Instructions

Make the spinach pesto: Place the spinach in a food processor with the walnuts, cheese, garlic, and nutmeg; season with salt and pepper. Turn on the processor and stream in enough EVOO to form a thick pesto.

Make the portobellos: Brush the portobello caps with a damp towel to clean them. In a large plastic food storage bag, combine the EVOO, Worcestershire sauce, balsamic vinegar, and rosemary. Add the mushroom caps and slush around to coat with seasonings.

Heat a grill pan or large skillet over medium-high heat. Shake the marinade off the mushrooms and cook the caps, turning once, 10 to 12 minutes, or until well browned on both sides. Season with salt and pepper.

Top the caps with the mozzarella, remove the pan from the heat, and tent loosely with aluminum foil. Let stand 1 to 2 minutes to melt the cheese.

Place the mushrooms on the roll bottoms and top with pesto, a mound of baby spinach, and a few slices of red onion.